In the electronic industry, for imparting solder receptivity to copper or copper alloy portions on electronic parts and circuits, it has been a common practice to form a tin, lead or tin-lead alloy (i.e., solder) coating on the copper or copper alloy portions by electroplating techniques As electronic devices are reduced in size, parts and circuits are also miniaturized or complicated. Some finely defined portions are difficult to plate by electroplating techniques. Of interest are electroless or chemical tin, lead or tin-lead plating techniques capable of plating on such finely defined portions.
Among electroless plating techniques for imparting solder receptivity, electroless tin plating has been used in practice. Electroless tin plating is generally intended for depositing thin films and uses a batchwise bath which is discharged without replenishment. In the case of electroless tin-lead alloy plating intended for depositing thick films, or if electroless tin or lead plating is intended for depositing thick films, it is necessary to maintain a constant rate of deposition. This necessitates replenishing chemical reagents as they are consumed.
Chemical reagents are generally replenished to the plating solution by analyzing the plating solution to measure the concentration of metal ion, calculating the amount of metal ion consumed (loss), and making up the reagent in an amount corresponding to the loss.
For assuring a constant rate of deposition, continuous replenishment of chemical reagents is necessary, which necessitates automatic monitoring of the concentration of relevant metal ion in the solution. However, lead and tin contained in tin, lead or tin-lead alloy plating baths are elements which are difficult to quantitatively analyze. Although several methods are known for measuring these elements, these methods have several problems. An analytical method which is accurate and adapted for automatic determination has not been available.
Analysis of lead ion is carried out by chelate titration using EDTA, precipitation of PbSO.sub.4 followed by measurement of the precipitate weight, or the like. The chelate titration technique suffers from low accuracy because it is very difficult to judge the end of titration The weight analysis technique requires cumbersome steps of filtration and drying and cannot be automated.
Analysis of tin ion is by reducing a tetravalent tin ion in a plating solution to a divalent tin ion and titrating the divalent tin ion with iodine or by chelate titration of a plating solution approximately at pH 5 using EDTA. The iodometric titration technique is not applicable to plating solutions containing another reducing compound. The chelate titration technique suffers from the same problems as above.
These analytical methods are difficult to apply to the bath management of tin, lead or tin-lead alloy plating solutions. There is a need for an analytical method which allows for accurate analysis of tin, lead or tin-lead alloy plating solutions for easy and consistent management thereof.